Wavelength division multiplexed passive optical networks, WDM PON, dramatically increase the capacity of fibre links compared to other fibre access techniques. Using dedicated wavelengths for each connection between a subscriber and the central office, the current technology allows 96 1 Gigabit Ethernet (GbE) connections to be established on the same optical fibre.
Several WDM PON system architectures and transmission techniques have been proposed to lower the cost or increase the performance of a WDM PON which require the use at the subscriber side of an optical network termination, ONT, that can be operated at any selected wavelength (a “colourless” ONT). There are basically two ways to implement a colourless ONT: sending the optical carrier signal to the ONT from a remote node (typically, the Central Office); and locally generating the optical carrier signal at the ONT by means of a tunable laser.
Many telecom operators are requiring the compatibility of WDM PON equipment with deployed PON infrastructure. A WDM PON ONT which is compatible with deployed PON infrastructure, which has distribution node simply comprising a power splitter, must comprise a tunable filter in order to select the respective working wavelength for each ONT, such as described by Murano et al, “Tunable GPON receivers enable phased migration to 1 Gb/s per subscriber”, OFC 2009. The transmission wavelength of the tunable filter must be set to the respective working wavelength for each ONT, which is currently done manually.